Fuel ethanol for motor vehicles is substantially generated using grain and corns at present in China. Large-scale production of fuel ethanol will results in competitive usage of foods, leading to rising food prices and potential food shortage. A possible solution uses non-food renewable biomass to generate ethanol. Kitchen waste comprises a huge amount of renewable biomass. Over 60 million tons of kitchen waste is produced every year in China, and most are used to feed livestock, or delivered to landfill or incineration, resulting in severe environmental pollution. Only a small part is utilized to for example make compost or biogas, but with low economic interest. Therefore, it is promising to use kitchen waste to produce fuel ethanol, which both converts wastes to valuables and relieves food and energy shortage crisis.
Kitchen waste is a nutrition-enriched renewable biomass containing over 95% organics such as starch, saccharides, proteins, lipids, vitamins and elements such as N, P, S, K, Ca, and Mg. This enables kitchen waste to be biologically reusable. However, high contents of water and nutrition in the kitchen waste make microorganisms propagate rapidly at room temperature, taking advantage of the organics and mineral salts, causing the waste to decay and stink which brings challenges to process.
Saccharomyces cerevisiae is industrially preferable strain for ethanol fermentation which is capable of efficiently convert glucose to ethanol. However, the strain can not naturally use the starch and proteins contained in kitchen waste as carbon and nitrogen sources to generate ethanol due to lack of enzymes for degrading starch to glucose and enzymes for degrading proteins to polypeptides and amino acids.
It is therefore necessary to introduce genes encoding enzymes degrading starch and proteins to the Saccharomyces cerevisiae so that the strain will be able to take advantage of the self-expressing amylase and protease to convert the starch and proteins in kitchen waste to carbon and nitrogen sources, such that the waste can be converted to fuel ethanol for industrial purpose.